Various catalysts have been proposed for fluorination processes which substitute fluorine atoms for chlorine. Frequently, these catalysts are oxides or halides of chromium, aluminum, cobalt, iron, titanium, nickel, copper, palladium or zirconium; which are unsupported or supported on active carbons or alumina.
The chromium catalysts are divided into three groups:
catalysts of chromium fluoride PA0 catalysts of chromium oxyfluoride PA0 catalysts of chromium oxide
Chromium oxide catalysts are described, for example, in British Pat. No. 896,068 and U.S. Pat. No. 3,157,707. These catalysts are obtained by the dehydration of chromium hydroxide or by the reduction of chromium trioxide deposited on activated alumina.
French Pat. No. 1,369,782 discloses catalysts comprising non-supported chromium oxides obtained by the dehydration of chromium oxide.
U.S. Pat. No. 3,258,500 describes a catalyst having a anhydrous non-supported trivalent chromium base that is prepared by the reduction of chromium trioxide by ethanol and activated by heating in an inert atmosphere of 400.degree.-600.degree. C.
Other patents such as Japanese Applications Nos. 70.116696 and 74.131610, teach the reduction of chromium troixide by aldehydes or hydrazine.
Catalysts having a black chromium oxide base, obtained by the thermal decomposition of chromium trioxide, chromic hydroxide or chromic carbonate in the presence of air or oxygen, are disclosed in French Pat. No. 1,358,997.
U.S. Pat. No. 3,978,145 teaches that the chromium oxide catalysts previously prepared are not amorphous, but have the crystalline structure of an orthorhombic .gamma.--CrOOH. The disclosure proposes catalysts having improved utility, comprising an hexagonal form of this hydroxide-oxide of chromium.
All of the chromium oxide catalysts of the prior art are more or less suitable for gaseous phase fluorination of aliphatic chlorinated derivatives in fixed bed reactor systems. In fluidized bed reactors, which require particles of regular form and homogeneous granulometry, the prior art catalysts are poorly adapted to fluorination processes. Simple grinding of the catalysts, followed by sifting for the selection of suitable-sized particles provides particles of irregular form and leads to a significant loss of the catalyst.
A further disadvantage of known chromium oxide catalysts is a weak resistance to crystallization that ultimately contributes to a shortening of their effective life span. Most of these catalysts yield a ratio relatively elevated in asymmetric isomers when they are used for the preparation of tetrachlorodifluoroethane, trichlorotrifluoroethane and dichlorotrifluoroethane. Ideally, the content of these asymmetric isomers should be as low as possible since they are more sensitive to hydrolysis and are, therefore, more corrosive.